The present invention relates to a device generating a precise reference voltage, more especially intended for producing, from an external supply potential which may vary between a minimum value and a maximum value, a precise reference output voltage which is stable regardless of the operating temperature of the generator and the value of the external supply potential.
Such generating devices are especially adapted to provide an electronic circuit, such as for example an analogue/digital converter, with a stable reference potential in such a way as to render the operation of this converter more stable and more precise, while also reducing the consumption of these generators.
Among these generating devices, the invention relates more especially to those comprising a semiconductor circuit 1, more commonly designated as xe2x80x9cbandgapxe2x80x9d circuit, this type of circuit making it possible to develop a reference voltage, hereinbelow designated semiconductor circuit 1, and at least one voltage multiplier circuit 2 arranged in cascade with this semiconductor circuit, this voltage multiplier circuit being intended for providing, from the reference voltage delivered by the semiconductor circuit, the stable reference output voltage. Such a generating device of the prior art is represented in FIG. 1a. 
Customarily, the semiconductor circuits of this type require, before any use, prior adjustment so that the reference potential delivered by the latter is as stable and precise as possible regardless of any variations in the external supply voltage and in the temperature.
The drawback of this xe2x80x9cbandgapxe2x80x9d semiconductor circuit 1 resides in the fact that a compromise must routinely be found between the obtaining of temperature precision and the obtaining of supply voltage precision. More precisely, the adjustment of this type of semiconductor circuit can be performed according to three schemes, that is to say:
either this semiconductor circuit is adjusted in such a way that the reference voltage delivered by it varies only, for example, by a few mV throughout the range of operating temperatures, to the detriment of a variation of, for example, several tens of mV throughout the range of the supply voltage;
or this semiconductor circuit is adjusted in such a way as to obtain a compromise between the temperature stability, the reference voltage delivered by it and the external supply voltage varying by, for example, about 10 mV voltage-wise and temperature-wise.
Such adjustment results in appreciable imprecision in the reference voltage delivered by this semiconductor circuit 1, this imprecision being, however, passed on through multiplication by the multiplying circuit 2 to the supposedly precise predetermined output voltage, delivered at the output of the voltage generating device.
Specifically, as represented in FIG. 1a, the voltage multiplier circuit 2 includes a differential amplifier OPA receiving on its negative terminal the reference voltage Vref as set-point voltage and a resistive feedback circuit Rxe2x80x21, Rxe2x80x22, Rxe2x80x23 with a decoupling capacitor C2 comprising a regulating transistor Tr connected between the supply voltage Vcc and the resistive bridge restoring in part the output voltage VOUT, supposedly precise reference voltage, on the positive terminal of the operational amplifier OPA. The gate electrode of the regulating transistor Tr is linked and controlled by the output of the differential amplifier OPA, the junction point between the regulating transistor Tr and the resistive bridge constituting the output terminal delivering the supposedly precise reference voltage. The regulating transistor Tr plays the role of a voltage-controlled resistor and the multiplier circuit 2 makes it possible to slave the output voltage VOUT to a value above the reference voltage Vref, but below the value of the supply voltage Vcc, as a function of the relative values of the resistors Rxe2x80x21, Rxe2x80x22 and Rxe2x80x23, the value of resistance of the regulating transistor Tr being low.
However, the variations in the supply voltage, and in the reference voltage Vref, are amplified as a consequence, thereby impairing the actual precision of the assembly.
Additionally these reference generators exhibit considerable consumption especially when the external supply potential Vcc is at its maximum value.
The object of the present invention is in particular to remedy these drawbacks by improving the precision and the stability of precise-reference generating devices, independently of their relative adjustment in terms of external supply voltage, respectively in terms of operating temperature, while also benefiting from lower consumption.
Accordingly, the device generating a precise reference voltage, which is the subject of the present invention, comprises a semiconductor circuit generating a reference voltage and a voltage multiplier circuit which are supplied from a supply voltage. The voltage multiplier circuit comprises at least one differential amplifier receiving on its negative terminal this reference voltage as set-point voltage and a resistive feedback circuit comprising a regulating transistor connected between the supply voltage and a resistive bridge restoring, in part, the precise reference voltage on the positive terminal of this differential amplifier. The gate electrode of the regulating transistor is linked and controlled by the output of the differential amplifier and the junction point between the regulating transistor and the resistive bridge constitutes, for this generating device, an output terminal delivering the precise reference voltage.
The device furthermore comprises a galvanic link linking this output terminal delivering this precise reference voltage to the supply input of the semiconductor circuit and an initialization circuit connected to the gate electrode of the regulating transistor and making it possible on initialization, by turning on at the supply voltage of this precise reference voltage generating device, to replace the precise reference voltage with the build-up voltage of the supply voltage. This makes it possible, on the one hand, under transient conditions, on initialization, to supply the semiconductor circuit from the build-up voltage of the supply voltage, and, on the other hand, under steady conditions, to deliver on the output terminal of this generating device the precise reference voltage and to supply the semiconductor circuit from this precise reference voltage.
The initialization circuit includes a circuit generating a control pulse of specified duration, this control pulse applied to the gate electrode of the regulating transistor tripping this regulating transistor into the fully on state, for the duration of initialization. This makes it possible to impose on the output terminal of the device generating a precise reference voltage a voltage equal to said build-up voltage of said supply voltage.